AFAIK one Steam Controller is included in the purchase of a Steam Machine already. Also, I find it unlikely that the Steam Machine is not going to work with other controllers, since the SteamOS works with pretty much any controller out there (Xbox, PS4/PS5, Switch, 8-bit Do, you name it).

Not sure what the author meant here. Maybe they're referring for the fact that you can't purchase a cheaper version of Steam Machine without the Steam Controller.